1. Field of the Invention
This invention relates to objective lens systems having multiple lenses of crystalline semiconductor materials such as silicon and germanium. The invention lens is particularly adapted for use in light weight portable FLIR (Forward Looking Infrared) surveillance systems.
2. Description of the Prior Art
Four element infrared objective lens systems having axially aligned optical elements of silicon and germanium are known and described in U.S. Pat. No. 3,321,264 to H. K. Sijgers et al., issued May 23, 1967. Lens systems of this type find particular application in scanning infrared systems. These systems typically employ a detector upon which an image is focussed by the invention lens system. The detector used is typically flat. Prior art lens designs are best suited for projecting an image on a curved field of curvature. The present three element infrared objective is designed to form a flat field of curvature.
Infrared objective lens systems used to project an image through a cold stop on to a flat detector forming an image plane in a cryogenically cooled space have the problem of re-imaging the detector onto itself. The mirror image of itself is due to reflections from the surfaces of lenses within the objective lens system. The image of the detector when sensed by the detector along with the intended image results in a loss of resolution. Re-imaging of the detector onto the detector by the infrared lens is an effect referred to as narcissus. The invention three element infrared objective lens is designed to effectively minimize the effect of narcissus while maintaining a flat field of curvature.